(a) Field of the Invention
This invention relates to a slit illuminating device suitable for reproducing an image original having a certain amount of thickness such as books, bound document, etc. used in a reproduction apparatus.
(b) Description of Prior Arts
Heretofore, illumination of a slit portion in the reproduction apparatus has mostly been done by focusing a light source image to the slit portion, as has already been known from FIG. 1 of U.S. Pat. No. 3,364,816. This method of illumination, however, has two problems. One is that, while the method effectively provides intense illumination to the slit portion, it brings about non-uniform illumination with respect to the slit having a limited amount of width, i.e., only the center part of the slit width, for instance, is intensely illuminated, while the side parts are less intensely illuminated, causing irregular luminance distribution. The other problem is that errors in manufacturing and fitting of each component elements in the illuminating device greatly affect the luminance distribution at the slit portion. For example, when the center position of the light source gets off its regular position, the luminance distribution at the slit portion varies to spoil its stability. To solve such disadvantage, there has been proposed a mechanism which broadly irradiates the slit portion with a converging light beam, as has been known from U.S. Pat. No. 3,982,116, whereby uniform luminance distribution can be obtained at the slit portion. While method of illumination does not cause the positional displacement of the light source to largely affect the luminance distribution at the slit portion, it is not so effective for illuminating an image original having a certain amount of thickness such as thick books, etc. The reason for this is that, when the thick book is opened and a page therein to be illuminated is placed on an image original mounting table with its page being faced down thereon to, the portion of the page near the bound side of the book slightly separates off the image original mounting table with the consequence that, when the slit portion is broadly illuminated with convergent light rays, the bound portion of the book is illuminated in only a narrow region.
Generally, a light beam which illuminates the slit portion direct from the light source and a light beam which is projected into the third reflecting mirror direct from the light source and reflected by the reflecting mirror to illuminated the slit portion are rendered deverging light rays. A Japanese Utility Model Patent Application Laid Open No. 53-140442 discloses an art, in which such direct light is taken into consideration in the luminance distribution at the slit portion. Besides the abovementioned two direct light beams, there are two other light beams, i.e., a light beam which emits from the light source and is reflected by a lower reflecting mirror to directly illuminate the slit portion and a light beam which emits from the light source, is reflected by an upper reflecting mirror, and is again reflected by third reflecting mirror to illuminate the slit portion, both of which form an image at the slit portion. The points of problem in the image forming illumination method with these light beams are as mentioned in the foregoing.
In case an image original having a certain amount of thickness such as thick books, etc., is illuminated it is desirable that these four light beams, including the direct light beams, be all divergent light rays for the broad illumination of the slit portion. The present invention has materialized this concept.